Mems resonator

ABSTRACT

Multiple degenerately-doped silicon layers are implemented within resonant structures to control multiple orders of temperature coefficients of frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Utility patent applicationSer. No. 18/072,506, filed Nov. 30, 2022, which in turn is acontinuation of U.S. Utility patent application Ser. No. 16/861,778,filed Apr. 29, 2020 (now U.S. Pat. No. 11,584,642), which in turn is adivisional of U.S. Utility patent application Ser. No. 15/676,890, filedAug. 14, 2017 (now U.S. Pat. No. 10,676,349), which in turn claims thebenefit of U.S. Provisional Patent Application Ser. Nos. 62/396,816,filed Sep. 19, 2016, and 62/374,675, filed Aug. 12, 2016. Theaforementioned patent applications are hereby incorporated by reference,as are each of the following:

Application No. Filing Date Title 61/937,601 9 Feb. 2014 MEMS Resonatorwith Improved Control of Temperature Coefficients of Frequency14/617,753 9 Feb. 2015 Temperature-Engineered Mems Resonator 62/181,76719 Jun. 2015 MEMS Heating and Superheating 62/183,689 23 Jun. 2015Temperature-Activated MEMS Frequency Trim 15/186,510 19 Jun. 2016Microelectromechanical Resonator

TECHNICAL FIELD

The disclosure herein relates to the field of microelectromechanicalsystems (MEMS) and more particularly to resonant MEMS structures.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments disclosed herein are illustrated by way ofexample, and not by way of limitation, in the figures of theaccompanying drawings and in which like reference numerals refer tosimilar elements and in which:

FIGS. 1A-1C illustrate various physical, electrical and TCF-engineeringaspects of an exemplary piezoelectrically-actuated MEMS resonator havingat least two degenerately-doped silicon layers;

FIGS. 2A and 2B illustrate generalized alternative processes forfabricating the dual-silicon resonator depicted in FIG. 1B;

FIGS. 3A-3U illustrate alternative approaches to forming adegenerately-doped silicon layer or composite degenerately-doped siliconlayer, generally but not necessarily on an insulator layer and handlewafer to form a degenerately-doped SOI structure as shown in FIG. 2A;

FIGS. 4A and 4B illustrate dual-silicon resonator embodiments having oneor more metal layers or silicides to improve electrode conductivityand/or serve as seed beds to improve material structuring of an internalpiezoelectric layer;

FIGS. 5A-5I illustrate non-exhaustive examples of dual-silicon resonatorshapes and designs;

FIGS. 6A and 6B illustrate alternative material stacks having the threelayers shown in FIG. 1A together with an additional metal layer;

FIGS. 7A and 7B illustrate embodiments of symmetric dual-siliconmaterial stacks that avoid surface-tension imbalance and resultantstructural warping;

FIGS. 8A and 8B illustrate exemplary wafer-bonded resonatorencapsulations formed by solder-bonding a lid wafer and device wafer;

FIG. 8C illustrates a generalized silicon bonding technique in whichcounterpart single-crystal silicon seal rings are fused, or asingle-crystal silicon seal ring and polysilicon counterpart are fused,or counterpart polysilicon seal rings are fused;

FIG. 8D illustrates an oxide-based bonding approach in which an oxidelayer (formed on the lid wafer is fused an oxide seal ring on thedevice-layer surface;

FIG. 8E illustrates an exemplary low-temperature wafer-bondingimplemented within a vacuum chamber;

FIG. 8F illustrates a face-to-face wafer bonding approach in which aCMOS wafer is bonded to a MEMS wafer instead of a passive lid/cap wafer;

FIG. 8G also illustrates encapsulation of a MEMS feature using a CMOSwafer, but through bonding of a MEMS wafer face to a CMOS waferbackside;

FIG. 8H illustrates another face-to-face MEMS-to-CMOS wafer bondingapproach, in this case with a TSV build into the MEMS wafer prior tobonding;

FIGS. 8I and 8J illustrates a two-phase (or multiple-phase) TSVimplementation in connection with face-to-face MEMS-to-CMOS waferbonding;

FIGS. 9A-9F illustrate various encapsulation strategies that do notrequire wafer-to-wafer bonding;

FIGS. 10A-10D illustrate various approaches for through-silicon-via(TSV) formation within an encapsulated (or to-be-encapsulated)dual-silicon layer MEMS resonator;

FIGS. 11A-11E illustrate various approaches for integratingelectrostatic-discharge (ESD) protection devices within an encapsulated(or to-be-encapsulated) dual-silicon resonator;

FIGS. 12A and 12B illustrate exemplary electrical interconnectionsbetween encapsulation-level TSVs and piezo-actuated resonator electrodesthat enable both package-level terminal interconnection (i.e., duringsubsequent packaging steps) and post-encapsulation joule-heating of theMEMS resonator;

FIG. 13A illustrates a post-encapsulation joule-heating configuration inwhich electrically conductive vias extend through a hermeticallyencapsulating lid to contact corresponding electrically conductiveanchoring structures;

FIG. 13B illustrates an alternative embodiment in which electricallyconductive vias extend through the device substrate to enablejoule-heating current flow through either or both of the resonatorelectrodes;

FIG. 14A illustrates an exemplary thermal profile achieved by conductinga DC or AC joule-heating current through a resonator structure viaanchors and folded tethers;

FIG. 14B illustrates an alternative heating arrangement in whichjoule-heating is sourced primarily within a patterned electrode layer ofthe dual-silicon resonator;

FIG. 15 illustrates an exemplary processing of packageddual-silicon-resonator MEMS device in which an oven reflow operation isexecuted to merge two separate terminals that form a joule-heating portinto a single operational terminal;

FIGS. 16A and 16B illustrate an alternative terminal reduction approachin which three or more encapsulation-level terminals are merged prior toenclosure within a package housing to expose only two electricallyindependent package-level terminals;

FIG. 17A illustrates an exemplary oscillator system having adual-silicon resonator in combination with a sustaining/sensing circuit;

FIG. 17B illustrates an alternative oscillator system embodiment inwhich the frequency of a dual-silicon resonator output signal ismodified within an output processing stage;

FIG. 18 illustrates various options for integrating a temperature-sensorwithin an oscillator system having a dual-silicon resonator;

FIG. 19 illustrates an exemplary process for fabricating a MEMSthermistor and dual-silicon MEMS resonator within neighboring regions ofthe same SOI substrate;

FIG. 20 illustrates an exemplary interconnect arrangement within adual-silicon resonator having an additional lightly-doped (or undoped)single-crystal silicon thermistor layer as part of the resonatorstructure;

FIG. 21 illustrates an alternative temperature-sensing embodiment thatexploits an output frequency difference between a temperature-sensitiveresonator and a relatively temperature-insensitive (“temperature-flat”)dual-silicon resonator; and

FIG. 22 illustrates an alternative temperature-sensing approach in whichtwo simultaneous and disparate temperature-dependent resonance modeswithin a singular dual-silicon resonator are exploited to yield atemperature measurement.

DETAILED DESCRIPTION

Temperature-stable, wear-resistant resonators formed by material stackshaving at least two degenerately-doped silicon layers and apiezoelectric material layer are disclosed in various embodimentsherein. In a number of implementations, the piezoelectric material layer(“piezo layer”) is sandwiched between a degenerately-dopedsingle-crystal silicon “core” layer and a degenerately-dopedpolycrystalline layer, with those outer silicon layers serving aselectrodes for conducting drive/sense signals to the piezolayer—obviating conventional metal electrode layers and their undesiredaging properties (i.e., wear-hardening over time). Further, as describedfor example and without limitation in incorporated U.S. patentapplication no. 61/937,601 and Ser. No. 14/617,753, layer thicknessratios, crystallographic orientation of the single-crystal layer (atleast), mode shaping and/or degenerate dopant concentration/type may beengineered to substantially zero at least the first-order (linear) andsecond-order (parabolic) temperature coefficients of frequency (TCFs)for the material stack as a whole, with the intrinsic parabolic TCF ofthe piezo layer, for example, being substantially canceled by areverse-polarity parabolic TCF engineered within the degenerately-dopedsingle-crystal silicon layer. Thus, in addition to the serving dual dutyas both a sense/drive electrode and the resonator bulk layer, thedegenerately-doped single-crystal silicon layer provides engineering“knobs” for yielding resonant MEMS structures having temperature-stableresonant frequencies (e.g., zero or near-zero combined TCF) over adesired operating temperature range (e.g., −45° C. to +85° C.).Similarly, the degenerately-doped polycrystalline silicon (“polysilicon”or “poly”) layer serves as a wear-resistant electrode (and thus may beviewed, along with the piezo layer as part of the sense/drive component)and may also enable multiple degrees of TCF engineering. Moreover, boththe poly layer and the single-crystal silicon layer enable thermalfrequency trim operations as described for example and withoutlimitation in incorporated U.S. patent application no. 62/183,689 andSer. No. 15/186,510, permitting targeted, post-encapsulation and/orpost-packaging adjustment of resonator frequency (e.g., adjustingnatural frequency of wafer-scale or singulated resonators to within 10ppm or less of a target output frequency). In other embodiments,piezoelectrically actuated, dual silicon-layer resonators areencapsulated with or without wafer-scale bonding, electrically completedaccording to various through-silicon-via arrangements, and fabricatedwith integrated electrostatic-discharge (ESD) protection circuits, anyor all of which may leverage one or more component layers of thedual-silicon material stack. In yet other embodiments, one or more insitu temperature-sense elements (e.g., implemented within the resonatorpackage, encapsulation chamber or even within the resonator structureitself), are provided to enable temperature-adjusted frequency-pulland/or post-resonator output frequency adjustment to further flatten thenet resonator TCF (i.e., sum of 1^(st)-order, 2^(nd)-order, . . . ,n^(th)-order TCFs) and/or compensate for aging affects or even permitrun-time frequency adjustment (e.g., by enabling closed-loop heating toa desired operating temperature that leverages a 0^(th)-order TCF).

FIGS. 1A-1C illustrate various physical, electrical and TCF-engineeringaspects of an exemplary piezoelectrically-actuated MEMS resonator havingat least two degenerately-doped silicon layers, referred to herein as a“dual-silicon” resonator. Referring first to FIG. 1A, the physical stackof materials shown in cross-section A-A′ includes a degenerately-doped(DD) single-crystal silicon (SC Si) core, a piezoelectric layer (adielectric), and a degenerately-doped polycrystalline silicon (DD poly)layer. Dopant concentrations within the outer silicon layers issufficiently high (e.g., greater than 1E19/cm³) to effect low-losselectrical conductivity and thus enable the silicon layers to serve aselectrodes for establishing a time-varying piezo-actuation voltageacross the piezoelectric layer, obviating more conventional metal layersthat tend to harden over time and undesirably shift the resonatorfrequency. In the particular embodiment shown, the top and bottomelectrodes implemented by the poly and single-crystal silicon layers,respectively (with “top” and “bottom” referring arbitrarily to thedrawing orientation as the structure may be flipped or otherwisereoriented in deployment), are electrically coupled to exterior packagecontacts through mechanical tether and anchoring structures to enableresonator reception of a piezo-actuating “drive” signal (V_(sustain))and enable external sensing of a piezoelectric output signal (V_(sense))indicative of mechanical motion of the resonant structure. As shown, theresonator material stack may be electrically modeled by a platecapacitance (the piezo electric layer) coupled to drive and sense nodesvia low-loss conductive paths.

FIG. 1B illustrates an exemplary cross section through anchors, tethersand body of a more detailed dual-silicon piezo-actuated MEMS resonator.As shown, the piezoelectric layer is implemented by an aluminum nitridefilm (AlN) which extends, along with the two degenerately-doped siliconlayers, through the tethers and anchors. Although depicted as disjointedelements, the tethers, anchors and resonator body are physically coupledto one another, with the tethers serving as spring-like structures topermit mechanical motion of the resonator (the “moveable micromachinedmember”) which is otherwise released from and suspended relative to thesurrounding field areas. Accordingly, conductive paths may be formedthrough the anchors and tethers to generate an electrostatic potentialacross the aluminum nitride piezo layer and thereby actuate theresonator, causing oscillatory mechanical motion thereof in one or moremechanical resonance modes (e.g., extensional, breathe, lame, flexural,or any other practicable resonance mode). Various alternative materialsmay be used to implement the piezoelectric layer in the embodiment ofFIG. 1B and all others disclosed herein including, for example and inisolation or any practicable combination, Zinc Oxide (ZnO), LeadZirconate Titanate (Pb[Zr_(x)Ti_(1-x)]O₃ 0≤x≤1), Lithium Niobate(LiNbO₃), Gallium Nitride (GaN), Indium Nitride (InN), Scandium AluminumNitride (ScAlN), Quartz (SiO₄), etc.

With respect to TCF engineering, the degenerately-doped single crystalsilicon layer incorporates at least two degrees of authority into asingle layer: crystal orientation and dopant concentration. Additionalauthority is available with respect to deposition of the piezoelectricfilm on the highly doped single crystal silicon. In particular, thethickness ratio of the degenerately-doped single crystal silicon layerand an aluminum nitride piezoelectric layer (‘x/y’) may beselected/implemented to yield substantially matched, but opposite-signsecond-order TCFs in those two layers, thereby canceling orsubstantially attenuating their combined parabolic contribution to thenet resonator TCF, and also providing some control (authority) over thethird-order resonator TCF. The preferred crystal orientation in thepolysilicon film may also affect the first- and second-order TCFs.Accordingly, as shown in FIG. 1C, at least the first-order andsecond-order TCFs of the dual-silicon piezo-actuated resonator of FIGS.1A and 1B can be engineered (controlled, manipulated) by varying thecrystal orientation of the degenerately-doped single crystal siliconlayer at a particular doping concentration, dopant type and layerthickness, the aluminum nitride layer thickness, and/or thedegenerately-doped polysilicon layer thickness at a particular dopantconcentration. In a number of embodiments, for example, the various TCForders are engineered to yield a substantially zero net TCF (e.g.,frequency within predetermined part-per-million (ppm) orpart-per-billion (ppb) of target) over a desired operating range (e.g.,−45° C. to 85° C.).

FIGS. 2A and 2B illustrate generalized alternative processes forfabricating the dual-silicon resonator depicted in FIG. 1B. In FIG. 2A,a degenerately-doped silicon layer (“DD Si”) is formed over aninsulating oxide (e.g., SiO₂), itself formed over a handling wafer“handle”, thus forming a silicon-on-insulator (SOI) structure in whichthe top silicon layer is degenerately doped. After forming an aluminumnitride piezoelectric layer (i.e., grown, deposited, etc.) over thedegenerately-doped silicon layer, a layer of in-situ-doped poly-silicon(ISDP) is deposited over the piezoelectric layer to complete thematerial stack. The stack is then etched to the oxide layer, formingisolation trenches that separate structural elements of themicromachined resonator system, followed by selective removal (e.g.,through vapor-phase etching with hydrofluoric gas) of oxide-layer(insulator-layer) material to release the resonator element and at leastportions of the tethering structures from the overall structure and thusrender the resonator element moveable relative to the surrounding fieldarea.

In FIG. 2B, process steps are reordered so that the piezoelectric layer(the “piezo layer”) and an un-doped (or lightly doped) polysilicon layerare formed over a conventional SOI wafer (i.e., wafer in which theworking silicon layer is not degenerately doped), and then etched toyield a structural arrangement similar to post-etch structure of FIG. 2Asans degenerately-doped silicon layers. Thereafter, a n-type dopantsource is deposited over the upper and lateral surfaces of thetrench-isolated structures—a dopant source shown in FIG. 2B asphosphosilicate glass (PSG), itself formed, for example, by exposure tovapor-phase phosphorous oxychloride (POCl₃). A thermal drive cycle isthen carried out (heat and time) to diffuse the phosphorous dopant intothe polysilicon and single-crystal silicon layers, followed by the sameHF-release (or other structurally releasing etch of the insulator layer)as in FIG. 2A to finalize the structural arrangement.

While the material stacks shown in FIGS. 2A and 2B and embodiments belowhave/are generally described in terms of phosphoric degenerate doping,n-type dopants other than phosphorous may be used in alternativeembodiments (e.g., arsenic, antimony, bismuth, lithium, etc.) andpredominantly p-type dopants (e.g., boron, aluminum, nitrogen, gallium,indium, etc.) may be used instead of n-type dopants, with correspondingchange in substrate type. Also, layer thicknesses depicted in FIGS. 2Aand 2B and those discussed below are not necessarily to actual scale andmay be disproportionate to actual scale. In a number of embodiments, forexample, the thickness ratio of the degenerately-doped single-crystalsilicon layer and aluminum nitride piezo layer is on the order of 30 to1, though layer thicknesses that yield greater or lesser ratios may beused. The degenerately-doped polysilicon layer thickness may be chosen,for example, to achieve viable top-electrode formation and providethermal frequency trim authority without disrupting the net TCFengineered within the piezo and degenerately-doped single-crystalsilicon layers. In a number of embodiments, for instance, thedegenerately-doped polysilicon layer thickness is roughly 25% ofsingle-crystal silicon thickness, though layer thicknesses in greater orlesser ratios may be implemented. In a particular embodiment, thedegenerately-doped single-crystal layer silicon layer is ˜6

m (microns) thick, the AlN piezo layer thickness is ˜0.2

m and the degenerately-doped polysilicon layer thickness is ˜1.6

m, with those thickness ratios (˜30:1:8) being approximately scaled(maintained) in taller or shorter material stacks. Any one or more ofthose material layers may be independently varied to achieve differentthickness ratios. Also, as in all embodiments herein, in-situ-dopedpolycrystalline silicon (ISDP) may be used in place ofdegenerately-doped polycrystalline silicon or as a starting point forproducing degenerately-doped polycrystalline silicon.

FIGS. 3A-3T illustrate alternative approaches to forming adegenerately-doped single-crystal silicon layer (or compositedegenerately-doped silicon layer), generally on an insulator layer andhandle wafer to form a degenerately-doped SOI structure as shown in FIG.2A. In all cases, an insulator wafer or cavity wafer may be used as thestarting substrate instead of an SOI.

FIG. 3A illustrates a degenerate-doping approach in which asingle-crystal silicon ingot 150 is grown with a desired andsubstantially uniform dopant concentration, then sliced to yielddegenerately-doped single-crystal wafers. Oxide deposition (on a surfaceof the degenerately-doped single-crystal silicon wafer—shown for exampleas SiO₂) and then bonding to a handle wafer (not shown) completes thedegenerately-doped SOI formation. Although a 10²⁰/cm³ dopantconcentration is depicted, concentrations may fall within theabove-specified range according to application requirements.

FIG. 3B illustrates another approach for achieving a substantiallyuniform dopant profile (as opposed to the dopant concentration gradientthat typically results in the diffusion-based approaches discussedbelow) in which a degenerately-doped single-crystal silicon layer isformed epitaxially over an insulator. In the particular example shown, aphosphorous-doped silicon layer is formed on an insulator (e.g., silicondioxide, sapphire or other insulating layer) within an epi chamber—otherdopant types may be used in alternative embodiments.

FIGS. 3C, 3D and 3E illustrate exemplary solid-phase, vapor-phase andliquid-phase dopant diffusion approaches, respectively, for fabricatinga degenerately-doped SOI structure. In the solid-phase example of FIG.3C, PSG is deposited on outer surfaces of a silicon-oxide-siliconstructure to yield twin degenerately-doped single-crystal silicon layersseparated by an oxide layer. Slicing through the oxide layer (anoptional process step) produces identical degenerately-dopedsingle-crystal silicon-on-insulator structures which may optionally bemounted on respective handle wafers.

In the vapor-phase dopant diffusion approach of FIG. 3D, dopant wafersare disposed adjacent respective surfaces of the single-crystal siliconsandwich shown in FIG. 3C (i.e., oxide sandwiched between outersingle-crystal silicon layers), and then batch-heated in a furnace oroven to release vapor-phase dopant and diffuse the gaseous dopant intothe single-crystal silicon layers of the structure. Slicing and optionalhandle-wafer bonding may be carried out thereafter as in the solid-phasediffusion example of FIG. 3C.

In the liquid-phase dopant diffusion example of FIG. 3E, a liquid-glassdopant is spun onto the surface of an undoped SOI structure, followed bya drive step to diffuse the dopant into the top single-crystal siliconlayer. Although liquid-phase dopant application (and also vapor-phaseand solid-phase) and diffusion is depicted with respect to asubstantially planar single-crystal silicon surface, etching or othertrenching operations may be carried out prior to dopant application inall cases to achieve multi-surface dopant diffusion (i.e., diffusinginto the top and side-wall surfaces of the etched single-crystalsilicon) and thus greater net dopant concentration within thesingle-crystal silicon layer. Examples of such tri-sided diffusion arediscussed below.

As mentioned, the dopant diffusion approaches shown in FIGS. 3C-3E tendto exhibit dopant concentration gradients, for example, peaking at thesurface of the doped layer and falling off rapidly at depths greaterthan ˜2 microns unless extra measures are taken for an increaseddiffusion depth. FIGS. 3F-3L illustrate various approaches fordiffusion-doping with increased net concentration and various differentgradient profiles. In the embodiment of FIG. 3F, for example, asingle-crystal silicon wafer is diffusion doped (solid-phase diffusionis depicted, though vapor-phase or liquid-phase diffusion are feasiblein this and other cases discussed below) to yield a surface-peakeddopant gradient as shown at 170. Thereafter, the wafer is flipped andbonded to an oxide-coated handle wafer to form an SOI structure with asubmerged degenerately-doped region (i.e., degenerately-doped regiondisposed adjacent insulator layer). The lesser-doped exposedsingle-crystal surface may then be ground or otherwise ablated to adesired depth to increase the net device-layer dopant concentration. Asshown by the dopant profile, the greater the grind/ablation depth, thegreater the net dopant concentration of the remaining silicon layer(i.e., as lower-concentration portions of the silicon layer areremoved).

FIG. 3G illustrates a degenerate doping approach similar to that of FIG.3F, but with the diffusion-drive executed after the flip-and-bondoperation (i.e., diffusing in situ within the SOI material stack), andFIG. 3H builds on the approach of FIG. 3G by depositing a second layerof PSG following the flip-and-bond operation so that dopant is diffusedinto both surfaces of the subject silicon layer in the ensuing driveoperation. As shown, the resulting dopant concentration profile in FIG.3H is essentially a superposition of mirrored instances of the profilesshown in FIGS. 3F and 3G, having a relative minimum at a center depth ofthe degenerately-doped silicon layer. As can be appreciated, thecenter-depth concentration will vary according to pre-doping thicknessof the single-crystal silicon layer, with the overall concentrationprofile tending toward uniformity as layer thickness is reduced.

FIG. 3I illustrates an alternative multi-surface dopant diffusionapproach in which the single-crystal silicon core layer is etched priorto dopant deposition, thus exposing the silicon side walls such that thelateral gradient profile (i.e., from left-to-right across thetrench-isolated single-silicon crystal structures) is similar to thatshown in FIG. 3H, orthogonally overlaid with thesingle-surface-diffusion profile as shown in FIGS. 3F and 3G. As shown,the trenches may be back-filled with epitaxial silicon, followed by adrive step to diffuse the dopant into the now-silicon-filled trenches(i.e., over-doping in the initial diffusion-drive to effect a target netconcentration on the subsequent backfill diffusion-drive). Agrind/polish step may carried out after the back-fill diffusion to yielda finalized degenerately-doped SOI structure, ready for dual-siliconresonator fabrication.

FIG. 3J illustrates a tri-sided diffusion approach similar to that ofFIG. 3I, except with degenerately-doped polysilicon back-fill (insteadof undoped single-crystal silicon back-fill), thus obviating the seconddrive step and yielding a hybrid core-layer combination ofdegenerately-doped single-crystal silicon and degenerately-dopedpolysilicon. FIG. 3K illustrates yet another tri-sided diffusionapproach, but with oxide back-fill instead of degenerately-dopedsilicon.

FIG. 3L illustrates another multi-lateral diffusion (tri-sideddiffusion), in this case from a degenerately-doped polysilicon source.After etching trenches on an SOI or SOC wafer (i.e., to form lightlydoped single-crystal silicon regions, “LDS” over a buried oxide layer,BOx, as shown at 190), highly doped polysilicon (ISDP over-doped to aconcentration of greater than 2e20/cm³ in this example) is deposited topartially or completely fill the trenches, thus creating adegenerately-doped crystal/polysilicon material. The initial doping ofthe polysilicon film is substantially higher than the final dopingconcentration so that, after a drive cycle, dopant from the over-dopedpolysilicon diffuses into the LDS to form degenerately-dopedsingle-crystal silicon (DD Si) together with polysilicon at a desiredfinal doping concentration (e.g. ˜2e20 cm⁻³ in this example).

FIG. 3M illustrates an approach similar to that of FIG. 3L, but withover-doped degenerately-doped silicon (“DD Epi Si”) epitaxiallyformed/grown over the trenched LDS (thus filling or at least partiallyfilling the trenches and forming an overlayer) instead of DD polydeposition. During the subsequent drive cycle, dopant from the episilicon diffuses into the LDS, shrinking the LDS cross section,potentially to zero, while the DD Si cross-section increases to adesired dopant concentration target.

FIGS. 3N and 3O illustrate lateral diffusion approaches in which amasking layer is formed (or left) over the top surface of the waferprior to deployment of a dopant source. In the embodiment of FIG. 3N,ISDP is deposited and driven as in FIG. 3L, thus doping the LDSexclusively from the sides (i.e., mask layer prevents top-downdiffusion). Accordingly, after CM P (or etch back or otherplanarization) and optional mask removal, a hybrid (potentiallysymmetric) structure remains, with alternating regions of DD Si and DDpoly, each with a desired/target dopant concentration. In the embodimentof FIG. 3O, degenerately-doped silicon is epitaxially grown in thetrenches (similarly to the approach of FIG. 3M, except that nosubstantial epi growth occurs on the oxide mask) and then driven to adegenerately-doped single-crystal silicon structure as shown.

FIG. 3P illustrates an alternative epitaxial approach in which a layerof degenerately-doped single crystal silicon is grown over thin LDSlayer (which sits on the buried oxide). In one embodiment, the epitaxialdegenerately-doped silicon (“DD Si epi”) is doped above a targetconcentration so that a drive cycle (to diffuse dopant into the thin LDSlayer) merges the LDS and DD Si epi layers into a degenerately-dopedsilicon layer having a desired/target dopant concentration.

FIG. 3Q illustrates a multi-layer diffusion approach in whichdopant-deposition, dopant-diffusion (drive) and epitaxial silicon layergrowth processes are iteratively repeated to yield the composite dopantconcentration profile shown. That is, the dopant deposition anddiffusion operations are repeated for an initial single-crystal siliconlayer and each new epitaxially formed single-crystal silicon surfacelayer, with the multi-layer structure being optionally ground as shownto yield a substantially uniform-dopant-concentration single-crystalsilicon layer. Though solid-phase dopant deposition is depicted,liquid-phase or vapor-phase dopant deposition may be carried out in allor any one or more of the dopant deposition iterations. Morespecifically, where advantageous to do so, different dopant depositionoperations (vapor, solid and/or liquid phase in any order) may beexecuted to effect the layer-by-layer production of degenerately-dopedsilicon.

FIGS. 3R, 3S and 3T illustrate alternative degenerate doping approachesin which dopant is implanted within the single-crystal silicon layer ordiffused through a porous silicon layer to enable deeper degeneratedopant concentrations and/or depth-controlled concentration gradients(i.e., concentrations peaking at selected depths). In the embodiment ofFIG. 3R, ion-beam dopant implant is followed by a thermal diffusioncycle to yield a degenerately-doped single-crystal silicon layer (e.g.,as part of an SOI structure) which may be ground or otherwise ablated toremove lower-dopant-concentration surface regions. In the embodiment ofFIG. 3S, vapor-phase dopant is diffused into a porous single-crystalsilicon structure (followed by a thermal drive cycle) to achievesubstantially deeper degenerately doped concentrations than with similarvapor-phase diffusion into non-porous silicon (i.e., as shown at 3D).Note that in either of the processes shown in FIGS. 3R and 3S a thermalcycle may be sufficient to render the porous silicon layer nonporous(i.e., melting the porous silicon) and/or high-temperature epitaxy orother process step may be carried out to transform the porous siliconlayer to a nonporous silicon layer. FIG. 3T illustrates an approachsimilar to that of FIG. 3Q, but with repeated dopant implantation cyclesinstead of repeated dopant depositions. In other embodiments,dopant-implant and dopant-deposition cycles may be alternated todegenerately dope respective single-crystal silicon layers.

FIG. 3U illustrates a two-step doping process starting from a PSG filmgrown using POCl₃ (although other methods can be used for the initialdoping as described generally in FIGS. 3A to 3T). Once the dopant filmhas been deposited on a silicon substrate, a short thermal drivetransfers a shallow dose into the silicon. The dopant film is thenremoved from the wafer, in this example by HF etching of the PSG, andthe dopant dose is redistributed or driven deeper during a subsequentlonger thermal drive (i.e., anneal). This two-step doping approach hasthe advantage of separating the total dopant dose from the final desireddistribution profile. More specifically, if the PSG dopant source wasleft in place for the entire long anneal, the source volume would play agreater role in the final dopant concentration and distribution,potentially resulting in less consistent concentration/distributionprofiles. In addition, the two-step approach allows a relatively thinsilicon dioxide layer to be used as a doping mask to create patternedundoped regions.

In each of the dual-silicon resonator embodiments described thus far,one or more metal layers or silicides may be introduced duringlayer-stack fabrication to improve electrode conductivity and/or serveas seed beds to improve material structuring of the piezoelectric layer(e.g., growing more ordered vertical grains of an aluminum nitridepiezoelectric layer). FIG. 4A illustrates such an approach with a layerof titanium nitride, molybdenum (or any other practicable metal orsilicide layers) formed over the degenerately-doped single-crystalsilicon prior to piezo layer formation, with the top electrodeimplemented, as before, by a layer of highly doped polysilicon. FIG. 4Billustrates a similar approach in which a metal deposition (e.g.,titanium, tungsten, etc.) over the degenerately-doped single-crystalsilicon layer is thermally driven to form a silicide layer (e.g., TiSi₂,WSi, etc.) for improved piezoelectric layer formation and improvedbottom electrode conductivity. Though not specifically shown, the layerstack shown in FIG. 4A may be re-ordered by reversing the positions ofthe degenerately-doped poly layer and metal layer. While such anarrangement may be susceptible to aging/work-hardening, top-electrodeconductivity may be improved without sacrificing thermal frequency-trimauthority afforded by the degenerately-doped polysilicon layer.

Despite depiction in rectangular cross section to this point,dual-silicon resonators disclosed herein may be implemented in variousshapes and designs, each with features advantageous for particularuse-cases, resonant modes and/or fabrication processes. FIGS. 5A-5Iillustrate non-exhaustive examples of dual-silicon resonatorshapes/designs, with each exemplary resonator implemented in thecrystallographic orientation (i.e., 45 degrees off-axis from thesingle-crystal silicon layer axis and/or predominant axis of thepolysilicon layer, though various other angular orientations may beselected according to TCF engineering objectives). Referring first tothe resonator shown in FIG. 5A, centrally-disposed bi-lateral tethers(springs) secure the otherwise freed resonator body to field-areaanchors. The enlarged resonator end-masses tend to limit (restrict)modal interactions outside a desired resonant frequency over a frequencyrange of interest.

Referring to the resonators of FIGS. 5A-5I generally, resonant motioncauses stress in selected directions across the resonator body in viewof the in-plane anisotropic single crystal silicon core, particularly inthe [110] axial orientations shown. Slots or vents (e.g., ˜7 micronswide) are provided within the resonant body to provide stress relief andfacilitate vapor-phase HF release—and also to enable lateral dopantdiffusion in at least in some processes (e.g., as discussed above inreference to FIGS. 2B and 3I-3K) and/or effect TCF engineering. Forexample, in the resonators of FIGS. 5B and 5C (and others), the locationand size of the slots are chosen to enhance dopant concentration inregions of high stress and, conversely, attenuate dopant concentrationin low-stress regions. In the resonator designs of FIGS. 5D, 5E, 5F and5G, vents in field areas (outside the resonator body) may be used torelease those regions during vapor-phase HF etching, thus formcantilevers or other structures that suppress/avoid field-areainterference with resonant behavior. The exemplary design in 5G includesa single centrally disposed anchor/tether from which release ventsextend and tuned to limit transmission of vibration to the resonatorbulk. The embodiments of FIGS. 5H and 5I illustrate centrally anchoredresonators with more than two released members (or appendages) extendingtherefrom. The resonator of FIG. 5H, for example, resonatespredominantly in extensional mode (with each of the point masses movingin-plane to and from the anchor), while the four beams extending outwardfrom the central anchor in the FIG. 5I embodiment resonate in flexuralmode (in effect, operating like a double-ended tuning fork). Variousother resonator core shapes, anchoring schemes, appendage counts,angular orientations, etc. may be implemented in alternativeembodiments, including embodiments that purposefully exhibit multiplesimultaneous resonant modes (e.g., having different TCF characteristicsas discussed below).

FIGS. 6A and 6B illustrate alternative material stacks having the threelayers shown in FIG. 1A (i.e., the two degenerately-doped silicon layersand piezoelectric layer) together with an additional metal layer. Morespecifically, in FIG. 6A, an additional metal layer is disposed betweenthe degenerately-doped polysilicon layer and the piezo layer and serves,at least in part, to enhance conductivity of the top electrode. In oneembodiment, for example, the metal layer serves exclusively as the topelectrode for the piezo layer (e.g., with field-area via contactdirectly to the buried metal layer), in which case thedegenerately-doped poly layer may serve primarily as authority forthermal frequency trim. In the embodiment of FIG. 6B, thedegenerately-doped poly layer is relocated to the opposite side of thepiezo layer (i.e., sandwiched between the piezo layer and core DDsingle-crystal silicon layer) thereby enhancing conductivity of thebottom electrode and maintaining the poly-layer thermal-trim authority.Though not specifically shown, an additional metal layer may also bedisposed adjacent the bottom surface of the piezo layer (i.e., two metallayers sandwiching the piezo layer) to minimize conductive loss withoutsacrificing the TCF-engineering authority provided by thedegenerately-doped single-crystal silicon core layer, or thethermal-trim authority provided by the degenerately-doped poly layer.Moreover, while the dual degenerately-doped silicon layers(single-crystal and polycrystalline silicon) are constituents of allmaterial stacks described thus far, in all cases (including embodimentsdiscussed below) either or both of those layers may be only lightlydoped, undoped or even omitted altogether, for example, where theircontribution to the resonator bulk, TCF-engineering,thermal-frequency-trim and/or other engineering or device-finishingauthority is unneeded.

As discussed above, layer thicknesses within the dual-silicon resonatorare generally non-uniform, and chosen, for example, to yield a desiredTCF characteristic (e.g., with the core single-crystal silicon layerbeing 10 to 30 times the width of an aluminum nitride piezo layer) andbulk resonator size, aspect ratio or other dimension. Due to theirdiffering coefficients of expansion and internal stresses, however, thematerial layers tend to warp when laminated into a composite stack,complicating downstream fabrication steps (and possibly producingunwanted modal characteristics or otherwise degrading runtimeperformance). FIGS. 7A and 7B illustrate embodiments of symmetricdual-silicon material stacks that avoid surface-tension imbalance (andresultant warping). In the embodiment of FIG. 7A, for example, twodual-silicon/piezo-layer material stacks are fabricated generally asdescribed in reference to FIG. 2A, with one of the material stacks beingflipped and bonded to the other, thus yielding a six-layer stack (or 5layer if the bonded polysilicon layers are viewed as a single layer)having a line of symmetry at the bond surface. Thus, the relativelythick degenerately-doped single-crystal silicon core at the bottom ofthe stack is balanced by a same-height (“h1”) degenerately-dopedsingle-crystal silicon core at the top of the stack. Electrically, thestack may be viewed as a having a top and bottom electrode (formed bythe counterpart degenerately-doped single-crystal silicon layers) withseries-connected capacitors (the two piezoelectric material layers)coupled between those electrodes and coupled to each other through thebonded degenerately-doped polysilicon layers—a series-coupled pair ofcapacitors, electrically equivalent to a single capacitive elementhaving twice the elemental capacitance. One advantage of this approach,in addition to stress balancing, is the relatively tall material stackprofile that results from the twin single-crystal silicon (coreresonator) layers—an arrangement that may meet large-profile designspecifications (e.g., 12-20 micron resonator heights or more) withoutrequiring undue process scaling or additional material layers.

FIG. 7B illustrates an alternative symmetric dual-silicon material stackin which a layer of bondable conductive material is formed between thecore single-crystal silicon layer and the piezoelectric layer, followedby formation of another such layer over the piezo layer (sandwiching thepiezo layer between two films of the bondable conductive material).After surface preparation (e.g., chemical-mechanical planarization),another instance of the core silicon layer is bonded to the materialstack, thus forming (absent the bottom insulator and handle wafer) asubstantially symmetric material stack having the same series-coupledcapacitor electrical model as the material stack of FIG. 7A. In analternative to the FIG. 7B embodiment, the piezoelectric layer may besandwiched between non-conductive bonding layers.

In a number of embodiments, the various dual-silicon resonatorstructures described above are encapsulated within a hermetically sealedchamber between a lid wafer and the handle wafer of an SOI substrate,with a seal ring encircling the chamber and serving to bond the lidwafer to the SOI substrate. FIGS. 8A and 8B illustrate exemplarywafer-bonded resonator encapsulations formed by solder-bonding the lidwafer and device wafer (i.e., SOI substrate or other composite waferarrangement, also referred to herein as the “resonator wafer”). In theembodiment of FIG. 8A, for example, an aluminum seal ring (“Al”) isformed on the lid wafer and a counterpart germanium seal ring (“Ge”) isformed on the resonator wafer (encircling the MEMS resonator structure)with the two seal rings heat-fused to form an aluminum-germaniumsolder-bond as shown (the respective dispositions of aluminum andgermanium on the lid and resonator wafers may be reversed). In FIG. 8B,a similar approach is used, but with a gold seal-ring (“Au”) beingheat-fused to a gold-tin counterpart (“AuSn”) to form a eutecticgold-tin solder bond. FIG. 8C illustrates a generalized silicon bondingtechnique in which counterpart single-crystal silicon seal rings arefused, or a single-crystal silicon seal ring and polysilicon counterpartare fused, or counterpart polysilicon seal rings are fused. In thelatter case, specifically shown in FIG. 8C, additional poly-to-polybonding interfaces may be provided to strengthen the wafer bond and alsocouple through-silicon-vias (TSVs) 225 in the lid to device contactpoints, thereby establishing electrical contact through the lid to thetop and bottom electrodes of the dual-silicon material stack.

FIG. 8D illustrates an oxide-based bonding approach in which an oxidelayer (e.g., SiO₂) formed on the lid wafer is fused an oxide seal ringon the device-layer surface. After oxide bonding, TSVs may be formedthrough the lid or SOI substrate as discussed in greater detail below.In yet other embodiments, low-temperature bonding may be implementedwithin a vacuum chamber as shown in FIG. 8E (e.g., within a backgroundatmosphere of helium or hydrogen gas that can be diffused out), thusavoiding temperatures that may otherwise disrupt pre-fabricated elementsof the resonator material stack. In the particular example shown, a ramand chuck arrangement is used pressure-fuse counterpart components ofthe seal ring.

FIG. 8F illustrates a face-to-face wafer bonding approach in which aCMOS wafer is bonded to the MEMS wafer instead of a passive orconventional lid/cap wafer. The CMOS wafer may incorporate complete andvarious integrated circuits (e.g., sustain/drive circuits, frequencysynthesizer, temperature compensation in the case of a MEMS oscillatorpackage, or any other circuit components useful in a chip packagecontaining a MEMS die) or may be limited to electro-static dischargeprotection (ESD) circuits or other limited complementary MOS (metaloxide semiconductor) arrangement. In either case, the wafer bond may beimplemented using aluminum pads and traces on the CMOS wafer with anappropriate counterpart material on the MEMS wafer. For example, asilicon-germanium (SiGe) alloy may be used on the MEMS wafer (i.e., toform an Al—SiGe bond) to allow higher temperature post-processing thanelemental germanium (which may also or alternatively be used) or anyother elemental or alloyed material or, more generally, any combinationof MEMS-side and CMOS-side materials that enable creation of a stablehermetic bond, eutectic or otherwise). In one embodiment, for example,an aluminum-silicon alloy (AlSi—more generally, Al_(1-x), Si_(x) where xis generally greater than 1%) is disposed/patterned on either the CMOSwafer or the MEMS wafer, and germanium (or other material that will forma eutectic or otherwise sufficient bond with AlSi to hermetically sealMEMS feature 250) is disposed on the counterpart wafer. More generally,any of the bonding approaches shown herein may be used to bond any classof wafers including, for example and without limitation, bonding oneMEMS wafer to another MEMS wafer. Note also that the aluminum-siliconalloy may be formed as part of the bonding process (e.g., firstdepositing a silicon film over an aluminum bond layer, followed by aninitial thermal drive to produce the desired AlSi bonding alloy, thenexecuting a second thermal drive under pressure to bond the AlSi patternto the germanium counterpart and thus produce a AlSi—Ge wafer bond).

Liquidus temperatures>˜430° C. can be achieved using a combination ofaluminum, silicon, and germanium. One approach to achieve liquidustemperatures between 450° C. and 500° C. in materials that are commonlyused and compatible with IC fabrication is to use an alloy of Al and Si(Si>1%) on one substrate corresponding with a second substrate with Ge.While such alloys are not commonly used, they are easily deposited usingsputtering or other forms of physical vapor deposition. Another approachwould be to make a stack of Silicon and Aluminum (distinct layers), andthen melt them at the liquidus of Al—Si (˜600° C.) prior to bonding at alower temperature. This ternary system would have a eutectic temperaturehigher than that of Al—Ge, but lower than that of Al—Si.

FIG. 8G also illustrates encapsulation of a MEMS feature 260 (e.g.,resonator or other microelectromechanical structure) using a CMOS wafer,but through bonding of a MEMS wafer face to a CMOS wafer backside. Insuch an embodiment, the CMOS wafer may be ground to its final thickness,have TSVs (through-silicon-vias) and backside metallization implementedin a bondable metal such that the backside of the CMOS (i.e.,circuit-less side) face the MEMS feature cavity. As shown, a solder orcompression bond may be used (e.g., including a eutectic bond achievedthrough heat and compression). As shown at 270, wafer bonding may becarried out such that multiple CMOS dies are bonded to a single MEMS die(e.g., hermetically sealing respective MEMS features implemented in theMEMS die) or, conversely, such that multiple MEMS dies are bonded to asingle CMOS die.

FIG. 8H illustrates another face-to-face MEMS-to-CMOS wafer bondingapproach, in this case with a TSV 285 built into the MEMS wafer prior tobonding. Under this approach, the CMOS wafer is bonded directly to theMEMS wafer with all (or a substantial portion of) package-levelinterconnects to the CMOS circuitry occurring through the MEMS wafer.

FIG. 8I illustrates a two-phase TSV implementation in connection withface-to-face MEMS-to-CMOS wafer bonding. In an initial phase, a portionof the TSV is implemented within the MEMS wafer, projecting, as shown at290, through the device and buried-oxide layers into the handle layer,but not all the way through the handle layer. After bonding the CMOSwafer to the MEMS wafer, the MEMS wafer is backside-ground to its finalthickness. Then, in a final (second) phase of the TSV implementation, avia structure 292 is formed at the MEMS wafer backside, projecting to apoint of contact (within a trench-isolated region of the handle layer)with the initial-phase TSV portion to finish the end-to-end TSV. Oneadvantage of the FIG. 8I (two-phase) TSV implementation isprocess/temperature compatibility with respect to the initial portion ofthe TSV. That is, the initial TSV portion can be fabricated using ahigh-temperature, pre-wafer-bond process (forming a blind metal via orsilicide via) not generally possible after bonding. The TSV is finishedin the second phase using the full-thickness CMOS wafer as a handle sothat processing is straightforward, permitting the MEMS layer to bethinned considerably (e.g., to a thickness of 50 Om or less). FIG. 8Jillustrates a more detailed view of an exemplary two-phase (ormulti-phase) via implementation, with features/layers color-coded perthe accompanying legend.

FIGS. 9A-9F illustrate various encapsulation strategies that do notrequire wafer-to-wafer bonding. More specifically, FIG. 9A shows thebasic integration of an aluminum nitride piezoelectric stack withthin-film encapsulation. As aluminum nitride is a high temperaturematerial, it is resistant to the temperatures of silicon epitaxy.However, the thin-film encapsulation process depends on the selectivityof epitaxial silicon growth to insulating layers. As the aluminumnitride film also serves as an insulator, the conditions of silicongrowth need to be selective so that the upper polysilicon electrode isnot shorted to the underlying silicon by unintended growth on thealuminum nitride sidewall. Some selectivity can be gained in the aspectratio and formation of the vents such that deposition preferentiallyoccurs at the top of the vent while starving the resonator cavity ofprecursors. An alternative is to use silicon diffusion in hightemperature hydrogen to collapse the vents so that the cavity is sealedand then a structural silicon layer can be grown on top of the thinsealed silicon layer.

FIG. 9B illustrates an alternative piezo-layer integration approach inwhich a thin, deposition-selective dielectric spacer (i.e., selective tosilicon deposition during epitaxy) is formed on the exposed aluminumnitride surfaces (“potential short”) prior to proceeding with thin-filmencapsulation. In one embodiment, the silicon and aluminum nitridesurfaces are oxidized forming silicon dioxide (SiO₂) and aluminum oxide(e.g., Al₂O₃) surfaces, respectively. The silicon dioxide can be removedat a higher rate than aluminum oxide using chemically selective and/ordirectional etches such as RIE (reactive ion etch) leaving a thinaluminum oxide sidewall dielectric spacer covering the aluminum nitrideedges. Subsequently this structure can be sealed using silicon epitaxy.In an alternative embodiment, a silicon dioxide layer is deposited overthe aluminum nitride and polysilicon stack, and then directionallyetched to create a silicon dioxide spacer to serve the same purpose asthe deposition-selective dielectric spacer.

FIGS. 9C and 9D illustrate formation of one or more microchannels at oneof the silicon/silicon dioxide interfaces—hollow channels or tubes thatallow the diffusion of HF gas for vapor etching of silicon dioxide. Themicrochannels can be formed using a variety of techniques such aspre-patterning a specific material such as a silicide forming metal, orproducing a pattern of oxide/silicon interfaces and then exposing themto a high temperature so that they form hollow channels by shrinkage andgas-phase silicon dissolution, respectively. However formed, themicrochannels can be buried in a subsequent deposition of oxide orsilicon and later exposed through a strategic vent hole that is far fromthe resonator cavity. HF-vapor etching of silicon dioxide can beperformed effectively through this network of microchannels as inthin-film encapsulation, but sealing is simplified because the ventsthat are filled are far from the cavity, and the microchannels aresealed almost immediately. The microchannels can also be sealed usingrapid silicon diffusion in hydrogen gas; an approach that avoidsexposing the structures in the cavity to traditional epitaxy precursorssuch as silane, dichlorosilane, and chlorine gas. In an alternativeembodiment, the microchannels are sealed using a chemical vapordeposition process other than epitaxy, for instance low pressure orplasma enhanced CVD of oxide, silicon, nitride, or tungsten. Becauseindividual microchannels are formed in an orientation orthogonal to theprimary vent opening (hole), it is difficult to transfer such reactiveprecursors to the resonator cavity. In this latter case, most of thestructural integrity is provided by a thick vent layer film, so thatsealing is relatively thin—this in contrast to the traditional layerstack thicknesses for thin-film encapsulation approaches in which thevent layer is substantially thinner than the seal layer. Because themicrochannel is formed first, the lateral (planar) dimension of the venthole and the thin-film encapsulation layer thickness be varied relativeto one another without conventional constraints.

FIG. 9E illustrates a vent layer implemented by a porous film (ofrelatively uniform porosity) rather than a solid film with discrete ventholes. In general, the porous film enables high diffusivity of HF vaporand low transport of silicon-based precursors at high temperature. Theporous film layer can be formed from a traditional porous silicon film,a polysilicon film that is sufficiently thin or has been engineered tohave some chemical attack at the grain boundaries, for instance bydoping, or a film grown with intentional nanoscale defects that can beopened up with a subsequent etch or thermal process. In any case, afterremoving the sacrificial oxide (e.g., using vapor HF), the film can besealed through various methods such as silicon epitaxy (classicthin-film encapsulation) or other deposited films such as LPCVD or PECVDoxides or polysilicon.

FIG. 9F shows an alternate structure in which vent holes are formed overa region having a very thin underlying sacrificial oxide—sufficientlythin that, after HF vapor etching, a microchannel region connectingtaller cavities is formed. The subsequent epitaxy seals the thin channelquickly to prevent significant precursor intrusion (transport) into thecavity. Same or similar features can also be used for LPCVD or PECVDsealing with oxide or polysilicon.

FIGS. 10A-10D illustrate various approaches for through-silicon-via(TSV) formation within an encapsulated (or to-be-encapsulated)dual-silicon layer MEMS resonator. In the embodiment of FIG. 10A, forexample, vias (“Via 1,” “Via 2”) formed through insulated passages inthe lid are bonded to field-area conductive layers which extend, forexample, through the tethers to bottom and top electrodes within theresonator material stack. TSVs may alternatively (or additionally)formed through the handle wafer (i.e., bottom wafer in SOI substrate)and routed to the field-area conductive structures.

FIG. 10B illustrates a more detailed example of lid-based TSV formation(showing in cross-section the insulated orifice and conductive TSVextending through the orifice) within the perimeter of wafer-bondingseal ring. As shown, wafer-level probes may be used to confirm contactintegrity and carry out operational testing and/or other pre-singulationoperations (e.g., joule heating and thus thermal frequency trim).

FIG. 10C presents a more detailed example of through-substrate viaformation, with TSVs formed within insulated openings in the handlewafer and extending at least to the oxide (insulator) layer. Thereafter,etched regions are of the single-crystal silicon layer (device layer)and oxide layer are fill are filled with conductive material (e.g., ISDPor metal) to form an electrically continuous path through the SOIstructure, or at least sufficiently through the SOI structure to makecontact with laterally extending conduction paths to the top and bottomresonator electrodes. As in the through-lid TSV arrangement of FIG. 10B,wafer probes may be used to confirm contact integrity and carry outoperational testing and/or other pre-singulation operations.

FIG. 10D illustrates a hybrid via arrangement, with one or more TSVsextending through the lid (e.g., as shown in FIGS. 10A and 10B) and oneor more other TSVs extending through the substrate (as shown in FIG.10C).

FIGS. 11A-11E illustrate various approaches for integratingelectrostatic-discharge (ESD) protection devices within an encapsulated(or to-be-encapsulated) dual-silicon resonator. In the embodiment ofFIG. 11A, an oxide mask layer (e.g., SiO₂) is formed over aTSV-penetrated lid wafer (with the oxide depicted as an under-lidfeature according to the eventual encapsulation orientation) and thenetched to form access paths to the TSVs and selected regions of thep-type lid. Thereafter, an ISDP layer is deposited over the oxide maskto establish contact with the TSVs and selected lid regions, and thenetched to electrically isolate the TSV conduction paths. At this point,highly-doped polysilicon (ISDP) is disposed adjacent the selectedregions of the p-type lid wafer so that a thermal drive cycle willdiffuse n-type dopant into the lid, in effect forming an n-p-nbipolar-junction transistor (BJT) with a disproportionately large baseregion (i.e., implementing a reverse-bias diode between each of the TSVsand the lid and thus, series anode-to-anode-coupled diodes between theTSVs). Accordingly, any applied voltage greater than the diode breakdownvoltage (plus the diode threshold voltage) will result in conductionthrough the series-coupled diodes, thereby limiting the potential thatmay develop across the TSV terminals and protecting the dual-siliconresonator (and particularly its sensitive piezoelectric layer) fromexcess applied voltage. From a fabrication standpoint, the ISDPdeposition used for TSV contact is leveraged to create ESD protectionwith limited additional process change (e.g., a mere oxide mask-layerchange to expose the lid wafer regions that will eventually become theBJT collector and emitter regions, and a thermal drive cycle which maybe applied in either case to ensure contact between the TSVs and ISDPlayer).

FIG. 11B illustrates an approach similar to that of FIG. 11A, exceptwith n-type dopant implanted (rather than diffused) into the p-type lidregion. This process facilitates reverse-order BJT fabrication in thecase of an n-type lid wafer; that is, p-type dopant may be implanted toform a pnp BJT (i.e., series cathode-to-cathode coupled diodes betweenthe TSV terminals which exhibit the same breakdown-plus-thresholdturn-on potential as the npn configuration). In either of the FIG.11A/11B embodiments, metal or silicide traces may be formed over thediffused (or implanted) collector/emitter regions to eliminate hot spotsand increase efficiency of the ESD protection circuit.

FIGS. 11C, 11D and 11E illustrate exemplary implementations ofESD-protection circuits in the device silicon layer or handle layer ofan SOI substrate. In the embodiment of FIG. 11C, PSG is deposited overan oxide mask and then thermally driven to create adjacent n-typediffusions, separated by a portion of the p-type device-layer silicon(i.e., an npn BJT as discussed above). ISDP contacts may be formedbetween the diffusion regions (collector/emitter) and respectivethrough-lid or through-handle vias to complete the ESD-protectioncircuit. In the embodiment of FIG. 11D, the piezoelectric layer (AlN inthe example shown) is patterned to include openings for later-depositedISDP contact to the p-type device-layer silicon, thus enabling formationof n-type collector/emitter regions (i.e., through a thermal drive cycleas described in reference to FIG. 11A). As in the embodiment of FIG.11C, the ISDP features (and thus the BJT) may be interconnected to TSVsin the lid and/or substrate to complete the ESD-protection circuit.

In the embodiment of FIG. 11E, a deep-trench isolation (DTI) process isused to form isolated regions in the device layer material stack afterpiezoelectric film deposition. A deep contact process is then used topenetrate the oxide layer of the SOI structure so that subsequent ISDPdeposition (and etching) enables contact formation penetrating from theISDP layer of the device material stack to the p-type handle wafer. Asin the embodiments of FIGS. 11A and 11C, a thermal drive cycle willdiffuse dopant into the p-type handle wafer to form the collector andemitter diffusions of an npn BJT and thus, upon coupling the packageTSVs to the respective n-type handle-layer diffusions, an ESD-protectioncircuit.

FIGS. 12A and 12B illustrate exemplary electrical interconnectionsbetween encapsulation-level TSVs and piezo-actuated resonator electrodesthat enable both package-level terminal interconnection (i.e., duringsubsequent packaging steps) and post-encapsulation joule-heating of theMEMS resonator. In the embodiment of FIG. 12A, three terminals areexposed at TSVs through the lid and/or substrate layer of theencapsulated dual-silicon resonator—two terminals coupled to oppositelateral ends of the top electrode (shown as an ISDP layer, though any ofthe alternative material layer configurations may be used) and oneterminal coupled to the bottom electrode (a degenerately-dopedsingle-crystal silicon layer in this example). By this arrangement, adirect or alternating current may be passed through the tetheringstructures of the resonator and top electrode layer, either or both ofwhich may be designed to superheat the resonator material stack (e.g.,heat the resonator material stack to temperatures ranging from, forexample and without limitation, 500°-1300° Celsius) and thus carry outany number of post-encapsulation annealing operations and/or thermalfrequency trim operations. The embodiment of FIG. 12B is similar to thatof FIG. 12A, except that the joule-heating terminal pair istether-coupled to opposite lateral ends of the bottom electrode layer(e.g., degenerately-doped single-crystal silicon layer) to operate thebottom tethering layer and/or bottom electrode layer as the superheatingsource instead of the top-layer tether/electrode elements. In yet otherembodiments, two joule-heating terminal pairs may be provided, with eachcoupled to opposite ends of a respective degenerately-doped siliconlayer. Also, as discussed in greater detail below, the joule-heatingterminal pair in either of the FIG. 12A/12B embodiments may be merged toform, together with the third encapsulation terminal, the resonatorsense/drive terminals discussed in reference to FIG. 1A. Alternatively,the joule-heating terminal pair may be driven by equipotentialresonator-drive signals to emulate an electrical coupling of theterminal pair. That is, instead of wiring the terminals together toestablish an equipotential at the lateral ends of the relevant materiallayer (and tethering layers), the terminals may be driven to anequipotential by a drive/sense circuit of an attached logic die.

FIG. 13A illustrates a post-encapsulation joule-heating configuration inwhich electrically conductive vias extend through a hermeticallyencapsulating lid to contact corresponding electrically conductiveanchoring structures (i.e., anchors and tethering elements formed fromor layered with electrically conductive material). By this arrangement,joule heating current may be driven through the top and/or bottomresonator electrode during post-encapsulation finishing operations, oreven post-production, including joule-heating in the field before orafter the MEMS device has been deployed within a host system. FIG. 13Billustrates an alternative embodiment in which electrically conductivevias extend through the device substrate (e.g., bulk semiconductor) toenable joule-heating current flow through either or both of theresonator electrodes. In both the FIGS. 13A and 13B embodiments, thehermetic encapsulation may, together with a perimeter seal ring or likestructure, enclose a vacuum cavity or a cavity filled with inert,thermally isolating material, thus limiting thermal radiation from theresonator to the substrate and lid structures.

FIG. 14A illustrates an exemplary thermal profile achieved by conductinga DC or AC joule-heating current through a resonator structure viaanchors and folded tethers. In one embodiment, conductive paths withinthe tethering structures are implemented with higher electricalresistivity than within the anchors and resonator electrodes so thatjoule heating occurs predominantly within the tethers, resulting inconductive and possibly radiant heating of the dual-silicon resonator(where joule heating may also occur). The tethers may also thermallyisolate the resonator body from the ambient or near-ambient temperature(To) anchors during joule heating so that, in a vacuum environment atleast, the nominal tether temperature at the tether's physical midpoint(i.e., between the anchor and resonator body) rises to T₀+T_(JH)/2, andthe resonator body is heated relatively uniformly throughout to atemperature T₀+T_(JH). Other temperature profiles (including thosehaving nonlinear gradients) may be produced in alternative embodiments.The resonator body may be slightly cooler than the hottest part of thetethers during joule heating due to radiative dissipation or gasconduction, but will usually be near the highest temperature of thetethers. The temperature profile can be accounted for in the design andthermal frequency trim process.

FIG. 14B illustrates an alternative heating arrangement in whichjoule-heating is sourced primarily within a patterned electrode layer ofthe dual-silicon resonator. The tethers may still be structured toprovide thermal isolation between the resonator and anchoring fieldareas, but are otherwise sufficiently conductive such that thepredominant joule-heating voltage drop (and power dissipation) occurswithin the patterned electrode layer. In the particular example shown,the patterned electrode (e.g. implemented in one or both of thedegenerately-doped silicon layers—single-crystal silicon and/orpolysilicon, with the latter arrangement depicted) includes a relativelynarrow conductive passage extending between larger bulk interconnectnodes. Due to its relatively low cross-sectional area (andcorrespondingly higher resistance), the narrow passage constitutes thepredominant source of ohmic (I²R) power dissipation within theconduction path traversed by the joule-heating current and thus thepredominant joule-heating source. As with the tether-based heatingarrangement of FIG. 14A, the resonator may be heated relativelyuniformly to a target temperature (e.g., T₀+T_(JH)) with limited thermalradiation from the resonator to the substrate and lid structures.

FIG. 15 illustrates an exemplary processing of packageddual-silicon-resonator MEMS device in which an oven reflow operation isexecuted to merge two separate terminals that form a joule-heating portinto a single operational terminal. That is, prior to oven reflow, thepackaged MEMS device includes three electrically isolated terminals, twoof which form a port for heating or superheating the dual-siliconresonator structure as discussed above in reference to FIGS. 12A and12B, while the other terminal forms a first operational terminal. Asrespective solder layers (or layers of other heat-soluble electricallyconductive material) of the two heating port terminals are renderedfluid during oven reflow and flow together, the two heating portterminals are effectively merged to form the second of two operationalterminals (i.e., converting the MEMS resonator device from athree-terminal device to a two-terminal device). This same approach maybe carried out with respect to the first operational terminal (e.g.,reflowing to merge distinct terminals that may constitute a secondheating port into the first operational terminal) or other terminals notshown.

FIGS. 16A and 16B illustrate an alternative terminal reduction approachin which three or more encapsulation-level terminals (e.g., as discussedin reference to FIGS. 12A and 12B) are merged prior to enclosure withina package housing to expose only two electrically independentpackage-level terminals. In the conceptual diagram of FIG. 16A, thejoule-heating terminals (joule-heating port) are coupled to an ISDPlayer of a dual-silicon piezo-actuated layer stack (thus permittingintroduction of joule-heating current through wafer probes or singulateddie probes), but may alternatively be coupled to the core single-crystalresonator layer or supplemented by an additional heating port coupled tothe core resonator layer (i.e., in a four-terminal encapsulationstructure).

FIG. 16B illustrates an exemplary sequence of packaging operations toyield the intra-package terminal merger (reduction) shown in FIG. 16A,showing cross-sectional and top views of the encapsulated structure orpackaged structure at each point in the sequence. Starting with anexemplary three-terminal encapsulated structure, the two joule-heatingterminals are merged by a metal deposition to form a unifiedtop-electrode terminal (or bottom-electrode terminal). Thereafter, anadditional packaging layer (e.g., polyimide or other non-conductivemolding) is disposed over the merged electrode terminals withthrough-vias and solder-ball sockets formed as shown to yield afour-terminal package in which two of the package-level terminals areelectrically common (per the terminal-merging metal deposition) to formtop electrode interconnect nodes, and a third of the four terminals iscoupled to the bottom electrode (or vice-versa). The fourthpackage-level terminal is provided primarily for mechanical stabilityand may be left unconnected (“no connect” or “NC”) or redundantlycoupled to the bottom or top resonator electrode. In yet otherembodiments, particularly where an oblong packaging structure isdesired, only two package-level contacts may be exposed—one for each ofthe two resonator electrodes.

Any of the various dual-silicon piezo-actuated resonators disclosedherein may be combined with additional elements to form a system withimproved temperature stability and/or other useful functionality. In theembodiment of FIG. 17A, for example, a dual-silicon resonator iscombined with a sustaining/sensing circuit to form an oscillator. Theresonant frequency of the oscillator may be modified in atemperature-dependent manner (i.e., as shown in FIG. 3A, a temperaturesensor output signal—indicative of temperature—is received within afrequency modifying element which, in turn, provides atemperature-dependent resonant-frequency control signal to thedual-silicon resonator and/or sustaining/sensing circuit) and to yield aresonant frequency with improved temperature stability compared to thatachievable with the dual-silicon resonator alone. For example, atemperature-dependent time-varying electrostatic field may be applieddirectly to the dual-silicon piezo-actuated resonator via the top andbottom electrodes discussed above, for example, to adjust or pull thefrequency of the resonator by manipulating the piezo-actuated mechanicalstress on the resonator in a temperature dependent manner. Alternatively(or additionally), the capacitance of a varactor or othervariable-capacitance element within the sustaining/sensing circuit maybe modified in a temperature-dependent manner to tune the frequency ofthe oscillator system (i.e., system including dual-silicon resonator andsustaining/sensing circuit).

In an alternative embodiment, shown for example in FIG. 17B, anoscillatory signal (clock signal) generated by a dual-silicon resonatoris modified within an output processing stage that includes atemperature sensor and frequency modifying element. For example, atemperature-dependent signal from a temperature sensor may be providedto a fractional-N phase locked loop (an example of a frequency modifyingelement) to enable generation of an output frequency more stable over atemperature range of interest than the raw resonator output signal.

In a number of embodiments, a temperature sensor as shown in FIGS. 17Aand 17B may be implemented within one or more component members of anencapsulated dual silicon resonator, or within a multi-die package thatincludes a dual-silicon resonator (MEMS) die and logic die. FIG. 18illustrates various options for such temperature-sensor integration,including temperature sensor implementation within the lid or devicelayer of an encapsulated dual-silicon resonator and/or within acollocated logic die that additionally contains the sustaining/sensingcircuit for driving the dual-silicon resonator into one or more resonantoscillation modes and sensing the resonant mechanical motion of theresonator. In the case of in situ disposition within the device layer ofan encapsulated resonator structure, the temperature sensor may beimplemented by a secondary micromachined structure, and morespecifically by a micromachined (or MEMS) thermistor structure that issubstantially released from the device-layer substrate and having anynumber of shapes that permit relatively stress/strain-free thermalexpansion and contraction (and thus limited mechanically-inducednonlinearity in the temperature dependence of the end-to-end thermistorresistance). Though not specifically shown, the end-terminals of theMEMS thermistor may be coupled to TSVs in the encapsulation structure(lid and/or substrate) and ultimately to package level contacts toenable a temperature-sense output signal. When implemented in the lidwafer or logic die, the temperature sensor may be implemented by asilicon, polysilicon or metal thermistor feature, p-n junction and/orother thermally-sensitive feature. In yet other embodiments, discussedin greater detail below, the temperature sensor may be implemented by apatterned trace and/or p-n junction within one or more layers of thedual-silicon resonator itself.

FIG. 19 illustrates an exemplary process for fabricating a MEMSthermistor and dual-silicon MEMS resonator (having twodegenerately-doped silicon layers and a piezoelectric layer as discussedabove) within neighboring regions of the same SOI substrate (i.e., to beresident within the same encapsulated device layer). As shown, an oxidelayer is deposited over an SOI substrate and then etched (e.g.,wet-etched) to yield a masked thermistor region. After PSG deposition(e.g., via vapor-phase POCl₃) and a mini drive cycle (relatively shortduration drive) to achieve a relatively shallow dopant diffusion, andthen a long anneal (relatively long and/or high-temperature drive) toachieve a deeper final dopant diffusion profile. At this point thesingle-crystal silicon device layer is degenerately doped (withprocess-dependent gradient) except in a thermistor region beneath theoxide mask, which remains relatively undoped. Remaining layers of thedual-silicon resonator are deposited (e.g., AlN and ISDP or any othermaterials according to variants discussed above), the oxide mask isremoved and trenching/etching and release processes are carried out tofinalize the two collocated MEMS elements—dual-silicon resonator andneighboring MEMS thermistor. By juxtaposing the MEMS thermistor anddual-silicon resonator within the same device layer, and within the same(eventually) encapsulated chamber and singulated die (e.g., withinpracticable lineal separation of each other), temperature gradientsbetween the thermistor and resonator that plague less proximaltemperature-sense/resonator arrangements are eliminated (or renderednegligible), increasing accuracy of the various temperature-compensationschemes shown in FIGS. 17A and 17B.

FIG. 20 illustrates an exemplary interconnect arrangement within adual-silicon resonator having an additional lightly-doped (or undoped)single-crystal silicon thermistor layer as part of the resonatorstructure. While this arrangement adds a layer to the resonator materialstack (in this case with the thermistor layer disposed beneath the dualdegenerately-doped silicon layers and piezo layer described above—anyother stack location may be feasible), the thermal coupling of thethermistor layer to the remainder of the resonator material stackensures zero or near-zero temperature differential between those tworesonator regions, again, avoiding error-inducing temperature gradientssuffered by less proximal arrangements. As shown, an additional pair ofsense terminals may be coupled across the thermistor layer (e.g.,through conductive structures in the tethering/anchoring structures toopposite lateral ends of the thermistor layer) and routed to externalcontacts of the encapsulated material stack and eventualresonator/thermistor package.

FIG. 21 illustrates an alternative temperature-sensing arrangement inwhich an additional temperature-sensitive resonator, “R1” (instead of orin addition to a thermistor), is provided (and implemented, for example,within the same device layer and encapsulation) together with logiccircuitry for ascertaining an output frequency difference between thetemperature-sensitive resonator and the relativelytemperature-insensitive (“temperature-flat”) dual-silicon resonator,“R2.” By mapping the frequency difference between the two resonators toa temperature differential, an extremely precise and resolutetemperature measurement may be obtained and used, as shown in FIGS. 17Aand 17B, to yield a temperature-corrected resonator output signal. Aswith the thermistor-based temperature sensor embodiments of FIGS. 19 and20 , collocation of the two disparate-TCF resonators within the samedevice layer (and eventual encapsulation and singulated die) avoidserror-inducing temperature gradients between the two structures.

FIG. 22 illustrates an alternative temperature-sensing approach in whichtwo simultaneous and disparate temperature-dependent resonance modeswithin a singular dual-silicon resonator are exploited to yield atemperature measurement (i.e., by mapping the frequency differencebetween the resonator modes to a temperature differential) and thus atemperature-corrected resonator output signal.

The various MEMS systems, devices, structures, components disclosedherein, as well as related circuitry (e.g., sustaining circuitry, sensecircuitry, drive circuitry, conditioning circuitry, control circuitry,etc.) may be described using computer aided design tools and expressed(or represented), as data and/or instructions embodied in variouscomputer-readable media, in terms of their behavioral, registertransfer, logic component, transistor, layout geometries, and/or othercharacteristics. Formats of files and other objects in which suchstructure and/or circuit expressions may be implemented include, but arenot limited to, formats supporting behavioral languages such as C,Verilog, VHDL, and Matlab, formats supporting register level descriptionlanguages like RTL, and formats supporting geometry descriptionlanguages such as GDSII, GDSIII, GDSIV, CIF, MEBES and any othersuitable formats and languages. Computer-readable media in which suchformatted data and/or instructions may be embodied include, but are notlimited to, non-volatile storage media in various forms (e.g., optical,magnetic or semiconductor storage media) and carrier waves that may beused to transfer such formatted data and/or instructions throughwireless, optical, or wired signaling media or any combination thereof.Examples of transfers of such formatted data and/or instructions bycarrier waves include, but are not limited to, transfers (uploads,downloads, e-mail, etc.) over the Internet and/or other computernetworks via one or more data transfer protocols (e.g., HTTP, FTP, SMTP,etc.).

When received within a computing system via one or morecomputer-readable media, such data and/or instruction-based expressionsof the above described structures, circuits and/or circuitry may beprocessed by a processing entity (e.g., one or more processors) of thecomputing system in conjunction with execution of one or more othercomputer programs including, without limitation, net-list generationprograms, place and route programs and the like, to generate arepresentation or image of a physical manifestation of such structures,circuits and/or circuitry. Such representation or image may thereafterbe used in device fabrication, for example, by enabling generation ofone or more masks that are used to form various device/circuitcomponents, features or structures in a device fabrication process.

Moreover, the various structures (for example, the structures of theMEMS device), circuits and/or circuitry disclosed herein may berepresented via simulations using computer aided design and/or testingtools. The simulation of the various structures and/or characteristicsor operations thereof may be implemented by a computing system whereincharacteristics and operations of such structures and/or circuitry, andtechniques implemented thereby, are imitated, replicated and/orpredicted via a computer system. The disclosed embodiments encompasssuch simulations of the exemplary structures and circuitry disclosedherein, and/or techniques implemented thereby.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the disclosed embodiments. In some instances,the terminology and symbols may imply specific details that are notrequired to practice those embodiments. For example, any of the specificnumbers of signaling or operating frequencies, component circuits ordevices, numbers and/or arrangement of structural layers, materialtypes, dopant types and concentrations and the like can be differentfrom those described above in alternative embodiments. Additionally,links or other interconnection between integrated circuit devices orinternal circuit elements, structural elements or blocks may be shown asbuses or as single signal lines. Each of the buses can alternatively bea single signal line, and each of the single signal lines canalternatively be buses. The term “coupled” is used herein to express adirect connection as well as a connection through one or moreintervening circuits or structures. Device “programming” or“configuration” can include, for example and without limitation, loadinga control value into a register or other storage circuit within theintegrated circuit device in response to a host instruction (and thuscontrolling an operational aspect of the device and/or establishing adevice configuration) or through a one-time programming operation (e.g.,blowing fuses within a configuration circuit during device production),and/or connecting one or more selected pins or other contact structuresof the device to reference voltage lines (also referred to as strapping)to establish a particular device configuration or operation aspect ofthe device. The terms “exemplary” and “embodiment” are used to expressan example, not a preference or requirement. Also, the terms “may” and“can” are used interchangeably to denote optional (permissible) subjectmatter. The absence of either term should not be construed as meaningthat a given feature or technique is required.

Various modifications and changes can be made to the embodimentspresented herein without departing from the broader spirit and scope ofthe disclosure. For example, features or aspects of any of theembodiments can be applied in combination with any other of theembodiments or in place of counterpart features or aspects thereof.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

1. (canceled)
 2. A method of fabricating an integrated circuit, themethod comprising: forming a microelectromechanical systems (MEMS)structure so as to have a layer of doped, crystal silicon; and providingfor two electrodes, including an electrode operable to induce the bodyto vibrate at a resonance frequency and an electrode operable to sensethe resonance frequency, wherein the resonance frequency ischaracterized as having each of a first-order temperature-dependentvariation and a second-order temperature-dependent variation; whereinforming comprises fabricating the MEMS structure such that an angularrelationship between a crystallographic axis of the doped, crystalsilicon, and such that at least one of dopant type, dopantconcentration, and thickness of the layer of doped, single-crystalsilicon, are selected so as to provide an absolute value of thefirst-order temperature-dependent variation and an absolute value of thesecond-order temperature-dependent variation that are each less than apredetermined level over a temperature range of at least one hundreddegrees Celsius.
 3. The method of claim 2 wherein the layer of doped,crystal silicon is predominantly single-crystal silicon layer, andwherein forming comprises degenerately doping the predominantlysingle-crystal silicon layer with phosphorus.
 4. The method of claim 2wherein forming comprises epitaxially-growing the silicon of the layerof doped, crystal silicon and doping the silicon so as to have a uniformdopant concentration.
 5. The method of claim 2 wherein the temperaturerange encompasses −45 degrees Celsius and +85 degrees Celsius, whereinthe angular relationship and the at least one are selected such that theabsolute value of the first-order temperature-dependent variation isless than a target parts-per-million (PPM) value of the resonancefrequency and such that the absolute value of the second-ordertemperature-dependent variation is less than the target PPM value of theresonance frequency.
 6. The method of claim 2 wherein forming comprisesforming the body so as to also have a piezoelectric layer.
 7. The methodof claim 6 wherein: the piezoelectric layer comprises an aluminumnitride (AlN) material layer; the AlN material layer and the doped,crystal silicon layer each are characterized as having a respectivecontribution to the second-order temperature-dependent variation; andforming comprises configuring the doped, crystal silicon layer, in termsof one or more of angular relationship, the dopant type, the dopantconcentration, the layer thickness, such that the contribution of thedoped, crystal silicon layer has a parabolic characteristic that isinverted relative to the contribution of the AlN material layer, andsuch that the net second-order temperature-dependent of the body fromboth of the AlN material layer and the doped, crystal silicon layer isless than 10 parts-per-million (PPM) throughout an operating temperaturerange of negative forty-five degrees Celsius through positiveeighty-five degrees Celsius.
 8. The method of claim 2 wherein formingfurther comprises modifying as design of the body by performing one ofadding material to the body and removing material from the body so as toadjust at least one frequency-related characteristic of the MEMSstructure.
 9. The method of claim 8 wherein: the method furthercomprises encapsulating the body within a chamber of the integratedcircuit; and modifying the body is performed after the encapsulating.10. The method of claim 2 wherein: forming comprises forming the body ona MEMS die; and the method further comprises electrically coupling thetwo electrodes with a circuits die, the circuits die to provide at leastone actuation signal to drive the vibration of the body and to generate,from a signal from the MEMS die representing the resonance frequency, anoutput timing signal, the integrated circuit thereby being configured tooperate as an oscillator integrated circuit.
 11. The method of claim 2wherein forming further comprises forming each of an anchor and a tetherthat couples the anchor to the body, in a manner such that each of theanchor and the tether have a first degenerately-doped silicon layer anda second degenerately-doped silicon layer, with an aluminum nitridematerial layer therebetween, such that the first degenerately-dopedsilicon layer of the anchor and the tether forms part of a first one ofthe two electrodes and such that the second degenerately-doped siliconlayer of the anchor and the tether forms part of a second one of the twoelectrodes, each in absence of a metal electrode layer extending betweenthe anchor and the body.
 12. The method of claim 2 wherein forming theMEMS structure further comprises: forming a lid layer in a manner so asto, relative to a substrate, hermetically seal the body relative to anatmosphere outside the integrated circuit; and forming a firstthrough-layer via in conductive relationship with a first one of the twoelectrodes and a second through-layer via in conductive relationshipwith one of the two electrodes, such that each through-layer viatraverses a same one of the lid layer and the substrate.
 13. The methodof claim 2 wherein forming the MEMS structure comprises forming the bodyto have a cross-sectional area and such that the cross-sectional area iscentrally-anchored relative to at least one of a lid or a substrate. 14.The method of claim 2 wherein forming the MEMS structure comprisesforming the MEMS structure to be a plate resonator.
 15. A method offabricating an integrated circuit, the method comprising: forming, aspart of a first die, a microelectromechanical systems (MEMS) structureso as to have a layer of doped, crystal silicon; and providing for twoelectrodes in the first die, including an electrode operable to inducethe body to vibrate at a resonance frequency and an electrode operableto sense the resonance frequency, wherein the resonance frequency ischaracterized as having each of a first-order temperature-dependentvariation and a second-order temperature-dependent variation; andelectrically coupling the first die with a second die, the second die toprovide at least one actuation signal to one of the two electrodes todrive the vibration of the body and to generate, from a signal providedby one of the two electrodes, an output timing signal as a function ofthe resonance frequency, the integrated circuit thereby being configuredto operate as an oscillator integrated circuit; wherein formingcomprises fabricating the MEMS structure such that an angularrelationship between a crystallographic axis of the doped, crystalsilicon, and such that at least one of dopant type, dopantconcentration, and thickness of the layer of doped, single-crystalsilicon, are selected so as to provide an absolute value of thefirst-order temperature-dependent variation and an absolute value of thesecond-order temperature-dependent variation that are each less than apredetermined level over a temperature range of at least one hundreddegrees Celsius.
 16. The method of claim 15 wherein the layer of doped,crystal silicon is predominantly poly-crystal silicon layer, and whereinforming comprises degenerately doping the predominantly poly-crystalsilicon layer with phosphorus.
 17. The method of claim 15 wherein:forming comprises doping a layer of crystal silicon so as to have auniform dopant concentration; the temperature range encompasses −45degrees Celsius and +85 degrees Celsius; and the angular relationshipand the at least one are selected such that the absolute value of thefirst-order temperature-dependent variation is less than a targetparts-per-million (PPM) value of the resonance frequency and such thatthe absolute value of the second-order temperature-dependent variationis less than ten PPM of the resonance frequency.
 18. The method of claim15 wherein: forming comprises forming the body so as to also have apiezoelectric layer; the piezoelectric layer and the doped, crystalsilicon layer each are characterized as having a respective contributionto the second-order temperature-dependent variation; and formingcomprises configuring the doped, crystal silicon layer, in terms of oneor more of angular relationship, the dopant type, the dopantconcentration, the layer thickness, such that the contribution of thedoped, crystal silicon layer has a parabolic characteristic that isinverted relative to the contribution of the piezoelectric layer, andsuch that the net second-order temperature-dependent variation of theMEMS structure is less than 10 parts-per-million (PPM) throughout anoperating temperature range of negative forty-five degrees Celsiusthrough positive eighty-five degrees Celsius.
 19. The method of claim 15wherein forming the MEMS structure comprises forming the body to have across-sectional area and such that the cross-sectional area iscentrally-anchored relative to at least one of a lid or a substrate. 20.The method of claim 15 wherein forming the MEMS structure comprisesforming the MEMS structure to be a plate resonator.
 21. A method offabricating an integrated circuit, the method comprising: forming, aspart of a first die, a microelectromechanical systems (MEMS) structureso as to have a layer of doped, crystal silicon, wherein forming furthercomprises providing a lid layer so as to, relative to a substrate,hermetically seal the body relative to an atmosphere outside the firstdie, and forming a first through-layer via in conductive relationshipwith a first one of the two electrodes and a second through-layer via inconductive relationship with one of the two electrodes, such that eachthrough-layer via traverses a same one of the lid layer and thesubstrate; and wherein forming comprises providing for two electrodes inthe first die, including an electrode coupled to the first through-layervia and operable to induce the body to vibrate at a resonance frequencyand an electrode coupled to the second-through-layer via and operable tosense the resonance frequency, wherein the resonance frequency ischaracterized as having each of a first-order temperature-dependentvariation and a second-order temperature-dependent variation; andelectrically coupling the first through-layer via and the secondthrough-layer view with a second die, the second die to provide at leastone actuation signal to one of the two electrodes to drive the vibrationof the body and to generate, from a signal provided by one of the twoelectrodes, an output timing signal as a function of the resonancefrequency, the integrated circuit thereby being configured to operate asan oscillator integrated circuit; wherein forming comprises fabricatingthe MEMS structure such that an angular relationship between acrystallographic axis of the doped, crystal silicon, and such that atleast one of dopant type, dopant concentration, and thickness of thelayer of doped, single-crystal silicon, are selected so as to provide anabsolute value of the first-order temperature-dependent variation and anabsolute value of the second-order temperature-dependent variation thatare each less than a predetermined level over the temperature range ofat least one hundred degrees Celsius.